Development of a display device using a thin-film light emitting element that emits light upon flowing current therethrough has been carried out actively.
When current flows through the thin-film light emitting element in which a single layer or a multilayer thin film formed using one or both of an organic material and an inorganic material is connected to electrodes, the thin-film light emitting element emits light. Such a thin-film light emitting element has advantages of low power consumption, miniaturization, good visibility and the like, and therefore, expansion of markets for the thin-film light emitting element has been expected.
By forming a light emitting element with a multilayer structure that has different properties for each layer, the light emitting element can emit light more efficiently as compared with a conventional, light emitting element (e.g., see non patent document 1: C. W. Tang et al., Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987)).
In the thin-film light emitting element having the multilayer structure, a light emitting laminated body including a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, an electron injecting layer and the like is sandwiched between a pair of electrodes. By applying a voltage to one electrode that is higher than that of the other electrode, the light emitting layer can emit light. Further, the hole injecting layer, the hole transporting layer, the electron transporting layer and the electron injecting layer may not be provided depending on an element structure.
In order to form the light emitting laminated body having the above structure, materials having properties that are suitable for the respective layers are selected. In addition, it is necessary to select materials having suitable properties for the respective electrodes, between which the light emitting laminated body is sandwiched.
Concretely, the electrode that applied with a higher voltage than the other electrode should be formed using a material with a large work function (approximately 4.0 eV or more) while the other electrode should be formed using a material with a small work function (approximately 3.8 eV or less).
However, since the materials for these electrodes must be selected in consideration of film properties, manufacturing methods, light transmitting properties, conducting properties and the like, the range of choices for the electrode materials has been narrowed now.